There are many different types of heat sources which are employed to bond various materials together. One of these sources is through the use of high-frequency electromagnetic energy which has found important commercial success in producing a desired end-product, particularly in the packaging industry. Electromagnetic energy at radio frequencies (RF) is used to efficiently heat and seal certain materials. However, it is limited to those materials which are referred to as dielectrics. A dielectric material is one in which it is possible to store electrical energy by the application of an electric field. The energy is recoverable when the field is removed. Dielectric heating is the result of the interaction of the electromagnetic energy with various components in the atomic or molecular structure of the dielectric material.
In the packaging industry, especially in medical packaging, RF energy in the range of about 27-60 megahertz (MHz) is frequently used for sealing and assembling a desired end-product. Utilizing a material's ability to convert RF energy to thermal energy that effectively heats the material interface to a suitable temperature and upon the application of pressure, very strong seals can be created in a short cycle time. RF sealing offers a variety of advantages including efficient heat generation, adequate bead formation to result in strong seals, a self limiting power input for some materials, absence of particulate matter formation during sealing process and relatively short cycle times especially for thicker sections of material.
Materials suitable for RF sealing typically exhibit high dielectric losses at the frequency of excitation and between ambient temperature and the heat seal temperature ranging as high as 310.degree. C. However, many packaging materials exhibit little or no RF losses, thereby making them unsuitable for RF sealing applications. For example some polymers are, at a minimum, not well suited for RF heating operations as the heat sealing either does not occur, or if it occurs, it does so only after inefficiently prolonged periods of time. In commercial production lines, a quick heat seal operation is generally preferred over a prolonged heat seal operation.
Although some materials do not lend themselves to RF sealing applications, they do exhibit excellent mechanical strength, optical clarity and gloss, good elastomeric properties, and high temperature or autoclaving stability. Therefore, these materials are potentially excellent candidates for delivery and packaging, particularly for medical applications as flexible containers having a minimal thickness of 2 mils and tubing having a maximum thickness of 50 mils. Flexible containers and tubing are utilized in the medical industry for containing and delivering parenteral solutions, dialysis solutions, frozen and ambient drugs, nutrition products, respiratory therapy products, blood and plasma. Because the containers are utilized to contain fluids or solids that may be introduced into a patient's body, it is necessary for the containers to be essentially transparent, flexible, essentially free of extractables and capable of maintaining the product contained therein under sterile conditions until the product is accessed or removed from the container. Similar to the containers, the tubing used for delivery of the product from the container to the patient also needs to be flexible, essentially free of extractables and capable of delivering the product under sterile conditions. The film from which these containers and tubing are constructed must also meet these requirements.
As set forth above, because the film materials will be processed into, for example, a flexible container that houses a medical product that is introduced into a patient's body, it is necessary that the film structure does not contain chemicals that can be extracted by the medical product or are likely to pass with the medical product into the patient's body. In this regard, eliminating or minimizing the use of film materials which potentially contain such chemicals is highly favorable.
In addition to these desirable medical packaging characteristics exhibited by these weak-RF or non-RF active materials, many of these materials are known to be compatible with several drug compositions and are adaptable to known printing methods for labeling purposes. Further, the disposal of these materials may be achieved by incineration as the resulting by-products are essentially free of inorganic acids. The recycling of some of these materials is also a possible answer to disposal or waste issues.
Different methods have been previously suggested to achieve the bonding of non-RF responsive materials. For example, the use of buffer or RF responsive materials between RF generating electrodes and the materials sought to be bonded has had limited success, as discussed in U.S. Pat. No. 4,857,129. Others have addressed the problem by incorporating RF responsive materials through copolymerization with the non-RF responsive polymer materials, as disclosed in U.S. Pat. No. 4,847,155. While RF heat sealing application has been achieved under this approach, the resulting product does not offer all of the characteristics often required in medical applications. Still others have coextruded materials which rely upon thick, outer layers of RF active materials, up to 85% of the total film thickness, or high content blends of RF active materials, up to 60% of the total composition, to render heat sealability to a non-RF active material.